The bibliographic details of the publications referred to by author in the specification are collected at the end of the description.
The gas/liquid interface of the lung is lined with a monomolecular layer comprising phospholipid, neutral lipids and specific proteins (surfactant proteins A, B, C and D, herein referred to as SP-A, -B, -C and -D, respectively). Collectively known as “pulmonary surfactant”, these compounds lower surface tension, decrease the work of breathing, and stabilise the lung by varying surface tension allowing alveoli of different sizes to co-exist.
Pulmonary surfactant phospholipids are synthesised by Alveolar Type II cells where they are stored in distinctive vesicles known as lamellar bodies. In response to a variety of stimuli, in particular physical distortion of the type II cells, the contents of the lamellar bodies are released into the hypophase, where they hydrate to form a 3-D lattice structure known as tubular myelin. The tubular myelin in turn supplies the monomolecular layer at the gas/liquid interface that possesses the biophysical activity.
The components of the monomolecular layer have a defined life and are constantly replaced. The disaturated phospholipids (DSP) are credited with reducing surface tension to the very low values thought to occur at low lung volumes, while cholesterol, the second most abundant pulmonary surfactant lipid, is thought to affect the rate of adsorption and the fluidity of newly released material. The system is extremely dynamic; in rats, dipalmitoylphosphatidylcholine, the main component of mammalian pulmonary surfactant, has a half-life of ˜85 minutes in the alveolus with as much as 85% taken back into type II cells and reutilised (Nicholas et al., 1990).
To date, four proteins, SP-A, -B, -C and -D have been shown to be uniquely associated with mammalian pulmonary surfactant. There is a general consensus that the extremely hydrophobic proteins (SP-B and -C) are functional components of the monomolecular layer, whereas the more hydrophilic protein, SP-A appears to be more involved in pulmonary surfactant homeostasis and host defence, and SP-D is solely involved in host defence.
The adult respiratory distress syndrome (ARDS) represents a severe, diffuse lung injury caused by either direct, via the airways, or indirect, via the blood, trauma. The hallmark of ARDS is a deterioration in blood oxygenation and respiratory system compliance as a consequence of permeability edema. Whereas a variety of different insults may lead to ARDS, a common pathway probably results in the lung damage. Leukocyte activation within the lung, along with the release of oxygen free radicals, arachidonic acid metabolites, and inflammatory mediators such as interleukin-1, proteases, and tumour necrosis factor results in an increase in alveolo-capillary membrane permeability. With the loss of this macromolecular barrier, alveoli are flooded with serum proteins, which impair the function of pulmonary surfactant (Said et al., 1965; Holm et al., 1987). This creates hydrostatic forces that further exacerbates the condition (Jefferies et al., 1988), leading to alveolar edema and a concomitant deterioration in gas exchange and lung compliance.
In the last decade, numerous methods for determining lung permeability have been assessed (Staub et al. 1990). Generally, these have relied upon detecting flux of radiolabels into, or out of, the lung. However, few have been applied clinically because of logistic problems with suitable scanners, stability, and specificity of the labels, and uncertainty over mathematical modelling (Staub et al. 1990). Further, lung damage, such as that induced by a noxious agent, has only been clinically detectable when sufficient damage has occurred for there to be changes in airway resistance or gas exchange. It is well accepted that this reflects relatively advanced lung disease.
Surfactant proteins are normally only found in appreciable amounts in the lung. In the airspaces, SP-A predominantly forms high molecular weight oligomers (˜650 kDa) with Stokes radii of ˜35 nm (Voss et al., 1988). Although mature SP-B, which associates as a low Mr (˜18 kDa) thiol dependent homo-dimer (Johansson et al., 1991), is normally intimately associated with complexes of surfactant phospholipid, (Longo et al., 1992), in vitro and in vivo studies in isolated type II cells suggest that at least some of the protein is secreted into the alveolus as hydrophilic, monomeric proprotein and processing intermediate with Mr of ˜45 kDa and ˜25 kDa, respectively (Weaver and Whitsett, 1989; Doyle et al., 1997).
In work leading up to the present invention, the inventors have unexpectedly found that serum pulmonary surfactant levels provide an extremely sensitive diagnostic marker of either lung damage, and in particular early stage lung damage, or a predisposition to the development of lung damage.